Place the sugar and 50 ml (1¾ fl oz) water in a small saucepan, bring to the boil, without stirring, and cook until it reaches 118°C (244°F) and has become a caramel. Remove from the heat and add the pistachios. Pour over a 30 × 20 cm (12 × 8 inch) tray lined with baking paper. Leave to harden and cool to room temperature, then crush, using a mortar and pestle, into fine crumbs.
This can be made 2 days in advance and stored in an airtight container in the freezer.